Add a util/exec interface for testing execs.

Peeled off from ip-per-service
no can do.    I run afoul of one of the rules in go:

Can't declare methods on pointer values
http://play.golang.org/p/AP6v9EUtSW

Can't have a value-receiver method for a pointer value
http://play.golang.org/p/AP6v9EUtSW

Cmd is returned as a pointer, so unless I force users to deref it, which is different than the standard lib, it's stuck.  This is new territory in Go's spec, so maybe I am just mis-comprehending it.